Two T cell receptors TCRalpha beta and TCRgamma delta exit. Both are encoded by variable, diversity and joining gene segments that undergo rearrangement to produce highly diverse polypeptide chains. These receptors appear to be inherently different in their development, phenotype, anatomical localization and recognition. The TCRalpha beta recognizes peptide antigens of 8-15 amino acids bound to MHC encoded class I and II antigen-presenting molecules. Recognition by the TCRgamma delta? is not well understood. These cells generally lack CD4 and CD8 molecules, and few examples of MHC class I or II restricted gamma delta T cells have been found. Here we investigate recognition by the gamma delta TCR and propose to elucidate two complementary examples of recognition. A remarkable stimulation of gamma delta T cells expressing only the Vgamma2 and Vdelta2 gene segments occurs in response to a mycobacterial antigen. It is estimated that the precursor frequency of this reactivity is from 1 in 50 to 1 in 2 human gamma delta T cells in peripheral blood. The antigen appears to be non-protein in nature since it is less than 0.5 kD, protease resistant, lacks absorbance at OD214 and ninhydrin reactivity. It requires antigen-presenting cells, but these cells may lack MHC class 1, class II or CD1 molecules. We propose to determine the chemical structure of the antigen causing this profound reactivity by a series of chemical purification steps and to identify the antigen-presenting molecule by making mAb against antigen-presenting cells that block stimulation. A second type of reactivity involves recognition of a larger, presumably protein antigen that is presented by members of the CD1 family of molecules. We will purify this antigen from mycobacteria by chemical approaches or by expression approaches from lambda gt11 mycobacterial library preparations. The first type of antigen reactivity appears to be germline mediated, whereas the second type is hypothesized to involve the highly diverse junctional regions and may correspond to a gamma delta version of conventional recognition. We will characterize the novel antigens and antigen-presenting molecules and the TCRgamma delta domains that mediate recognition in the two cases. These basic studies to define the nature of gamma delta TCR recognition are an essential step in unraveling the biology of gamma delta T cells and would provide a sound basis for understanding their role in the immune system.